A conventional automatic tier management function technique (Patent Literature 1 and Patent Literature 2) is to: migrate pages to an upper tier when frequency of I/O to/from the pages in a certain cycle (measurement cycle) is high (hereinafter referred to as “promotion”); and migrate pages to a lower tier when the frequency of I/O frequency to/from the pages is low (hereinafter referred to as “demotion”).
Batch jobs operating, for example, on week days (from Monday to Friday) and not operating on weekends (Saturday and Sunday) are considered with regard to the above-described technique. If a monitor measurement cycle is shortened (to several hours) and if the number of I/Os decreases on weekends, target pages of the batch jobs are demoted. Then, the phenomenon of performance degradation occurs every Monday when the number of I/Os increases again. In order to avoid demotion for the above-described phenomenon, the conventional technology increases the monitor measurement cycle to several weeks and measures the frequency of the number of I/Os during the long cycle, thereby smoothing the number of I/Os and avoiding the demotion on weekends.